Coaxial antenna assembly



July 20, 1954 c SHANKS 2,684,399

' COAXIAL ANTENNA ASSEMBLY Filed March 17, 1950 IN VEN TOR.

Patented July 20, 1954 UNITED STATES PATENT OFFICE Claims. 1

This invention relates to a coaxial antenna assembly for transmitting and receiving electro magnetic radiations. The invention is particularly useful for high frequency radio transmitters and receivers exposed to the weather such as used by forest ranger stations, police stations, and other closely knit organizations employing high frequency radio communication between different units of the organization.

In antennae of the coaxial type to which the present invention relates, there are three primary members, as follows: a coaxial transmission line having a center conductor and a shielding braid, said line being referred to herein as the cable; an upper radiating element termed the whip connected to the center conductor of the cable; and an outer circular conductor of metal, of a length predetermined in accordance with the wave length desired to be transmitted, coaxially arranged with respect to the cable and electrically connected to the shielding braid. Such outer conductor is sometimes designated as the lower radiating element of a dipole antennae construction, and is frequently referred to herein and in the art as a skirt. of the length of the skirt may be from twenty inches to about seven feet, depending on the wave length to be transmitted. There is conventionally provided, as a support or frame upon which to mount the members, a hollow shaft or tube, referred to herein as the support member disposed interiorly of the skirt with the cable being received therein. The support member is usually made of metal for both structural and electrical reasons, although it could be made of dielectric material if of suitable structural strength. The antenna is erected vertically, and the skirt and pipe are joined together at their upper ends in concentric relation by a metal connection which i usually made to function as a seal to close the ends to the passage of atmosphere, and to form an electrical path thereacross. They are insulated, however, from the coaxial cable center conductor. The support member projects outwardly from the bottom of the skirt for a substantial distance, and is attached to a wall, water pipe, or other object to support the antenna.

In view of the fact that the skirt usually has a length of several feet coextensive with the support member, the connection between the support member and skirt at their top ends is not of sufiicient structural strength to maintain the bottom end of the skirt in concentric relation with the support member, as must be done to The rangeinsure proper radiating characteristics of the antenna. Eccentricity between the support member and skirt may result from bias efiects introduced by the connection of the two members at the top; lateral movement of the skirt caused by driving force of the wind; by the force of gravity, in the event the antenna is not erected in plumb; or possibly by other forces. It is, therefore, necessary to provide insulating spacing means between the bottom end of the skirt and the support member. This is conventionally done by employing spiral insulators, spaced washer discs, spoked insulators, etc., between the support member and the skirt.

Antennae of the above described class are generally used for outdoor installations where they are exposed to the weather. It will be seen from the above description that the support member and skirt form a pocket to which the air has access. The air carries moisture at all times, with the relative humidity sometimes ranging as high as Upon change in the moisture concentration of the air, or in the temperature of the ambient atmosphere, the moisture condenses on the interior wall of the skirt and on the exterior wall of the support member. The moisture in contact with the metallic surfaces of the support member and skirt causes slow corrosion of the metals and results in the formation of possibly solid corrosion substance, and, in any event, an electrolytic solution. The corrosion substance or the electrolytic solution descends under the force of gravity and either falls directly on the insulating spacer member or is attracted to it by surface wetting phenomena. A conducting film is thus formed constituting in effect a bridge over the surface of the insulator, resulting in an electrical leak. The efiiciency of the insulator spacer is then reduced, and, if the leak is of serious enough proportions, the radiating characteristics of the antenna are altered and impaired.

In geographical localities where the air is polluted with noxious gases of an acidic nature, such as sulfur dioxide and sulfur trioXide, the corrosive action of the moisture condensate is correspondingly greater. Also, in geographical localities where the atmosphere is laden with dust and foreign particles, such dust and foreign matter may settle on the dielectric spacer element and contribute to the leak effect.

In view of the fact that the space between the support member and skirt is restricted in the transverse dimension, internal spacing members are subject to the further disadvantage that they a? is are limited in size and it is difficult to provide both a long insulating path and the necessary structural strength to withstand the lateral forces encountered. Other principles of construction have been proposed by the prior art for excluding moisture from antennae, such as providing a hermetical seal as disclosed in Letters Fatent No. 2,451,258 to Trevor, or the filling oi the space between the various conductors with dielectric material, as disclosed in Letters Patent No. 2,478,313, to Peterson.

The solution to the problem proposed by the present invention resides principally in two features, (l) the provision of insulator means disposed exteriorly of both the skirt and support member units of the assembly, and (2) the making of the antenna assembly of detachable construction to permit disassembly thereof, thereby permitting occasional cleansing of the interiorsas the occasion warrants. The latter feature has the separate advantage of enabling the substitution of skirts of different lengths to change the resonant frequency of the antenna system. In accordance with the present invention, an antenna free from the problems and disadvantages mentioned above may be constructed at much less cost and with less en ineering control than is obtainable with the solutions heretofore proposed; also, a more positive solution of the, problem is provided, while providing for increased diversity in the utility of the antenna.

Itis, accordingly, an object of the present in vention to provide a novel dielectric spacer element for maintaining the skirt and support member of a coaxial antenna assembly in concentric relation. It is a further object of the invention to provide such novel spacer assembly of a design such that it may be made to provide a long dielectric path between the members to be electrically spaced from eachother and at the same time provide rigid lateral support therebetween.

It is a further object of the invention to provide an antenna construction which may be quickly assembled and disassembled for cleaning, changing frequency of resonance, or for other purposes.

t is a still further "object to achieve the above objectives with simplicity of design and economy of cost.

Other objects, features and advantages of the nvention will become apparent from a study of the following specification, taken in connection with the accompanying drawings, which illustrate a preferred embodiment of the invention; wherein like reference characters designate corresponding parts throughout the several views:

and wherein:

Figure 1 is a perspective view of an antenna constructed in accordance with the principles of the present invention;

Figure 2 is a longitudinal sectional view;

Figure 3 is a sectional view taken on the line 33 of Figure 2, but on reduced scale; and

Figure/l is a sectional view taken on the line 4-4 of Figure 2, also on reduced scale.

Referring to the drawings, it will be seen that the antenna comprises, as hereinbefore indicated, a coaxial cable iii, a supporting member illustrated in theform of a pipe !2, a lower radiating element or skirt Ill of a length less than the length ofthe support member, and an upper radiating element called the whip. The support member and skirt are detachably secured to gether at their top ends in concentric relation by a construction, designated generally in Figure 1 by the numeral 18, and referred to herein as spacing the head. The whip is detachably connected to thehead by a threaded fitting 28, as shown. The skirt and support member are concentrically spaced from each other at the bottom end of the skirt by a dielectric spacer assembly, generally designated in Figure 1 by the numeral 22. For the purpose of discussion of the insulator assembly, the support member may be considered as an inner conductor and the skirt as an outer conductor. A bracket 24 is shown in'Figure 1 having a clamped connection to the pipe I2 for supporting the base of the antenna by attaching the sameto a vertical water pipe 25, as illustrated in fragment, or the bracket may be used to support the antenna by securing the same to a wall or other fixed object. If the latter manner of installation isused, the clamp member as of the bracket is discarded and the bracket 24 is attached to the wall by means of lag screws or the like in lieu of the clamp screws 2! shown. The top of the antenna is designed to be maintained plumb by means of guy wires (not shown) to be attached to the ears 28 on the lower side of head l8.

Referring now more particularly to Figure 2, it will be seen that the head construction, designated generally by the numeral 18, comprises a metal bushing member 30 having an annular longitudinal wall 300:. of a thickness sufficient to fit snugly between the support member l2 and skirt M. The bushing is provided with a shoulder 3% adjacent the cylindrical wall 30a for seating engagement upon the top of the skirt. The bushing has a lateral annular flange 300 at its upper end, and also a central recess 30d for purposes explained hereinafter. The bushing is assembled by inserting the annular wall 3901. between the support member and the skirt, fixedly securing the inner edge of the same to the support member as by meansof a welded joint 32. When the support ,is made of aluminum, as is preferable for structural reasons, aluminum heliarc welding is preferably employed. The skirt M is removably secured to the exterior of bushing wall 30a by any suitable connection, such as screws 34. The bushing also comprises as an integral, part thereof three equi-spaced ears 28, already mentioned.

The coaxial cable is of conventional design and construction, and. comprises, in the form illustrated, an inner stranded conductor 46,2. molded insulation layer -32 of suitable dielectric material, a copper braid shielding 44. and an outer, moisture-proof covering 46 of insulating material. The metallic braid shield 44 has its exposed end bent over and fastened by soldering to a metallic washer 48 disposed in the recess 38d, which is in turn secured to the bushing 30 by screws 59. In this way, a radio frec uency connection is provided between the metallic braid M and the skirt.

The recess afld in top of bushing 30 is sealed, and the entire bushing assembly is insulated from the center conductor 4!! of the coaxial cable by means of a gasket 66 and cap 62, made of suitable dielectric material preferably of a phenolic type plastic. The function of the cap is to provide an insulated but rigid support for the whip iii. the details of construction being as follows: The cap has a circular recess 6211 on its under side in which is received the head 66a of stud bolt 55. Nut 68 holds the bolt securely in place.

Threaded fitting 28 attached to the lower end of the whip secures the whip to the stud bolt.

The stranded center conductor 40 of the coaxial cable is electrically connected to the 'whip 5 by being received in the bore 661) in the head of stud bolt 66, and soldered thereto.

The cap and its assembled parts are secured to the bushing by means of screws Ill with the gasket 6!} being disposed intermediate the two members. The function of the gasket 89, and the reason for making a sealed connection between the bushing wall 39a and the support member, is to exclude moisture and the atmosphere from the cable I d. To this end, the recess 3011 is also preferably filled with Wax before attaching cap 62.

It will thus be observed that the head. assembly I8 is so constructed and arranged as to enable easy assembly and disassembly of the antenna. Threaded fitting 26 and nut 88 may be removed, and then removal of screw bolts 10 enables the cap 62 to be lifted. The soldered connection between the stranded conductor 4t and the stud bolt 55 may be broken, if desired to remove bolt 66. There is no need for removal of the bushing 39 from the support member, and, consequently, welding has been used for making the connection therebetween, as shown.

The cable it is maintained in coaxial relation to the pipe 52 at its upper end by the manner of construction above described, and is coaxially supported in the lower end of the pipe by a connector, designated generally by the numeral H, of suitable design. The connector is positioned in the pipe so as to draw the cable taut. However, when the support is made of metal or of any material except one of high dielectric capacity, the disposition of the cable on the interior of the support member is not materially significant as maintenance of the support member in concentric relation with the skirt is sufficient to control the radiating characteristics of the antenna. The connector is provided principally for convenience and economy to avoid the use of long coaxial cable leads, as otherwise the long lead on the coaxial transmission line itself might be guided through the pipe and connected directly to the whip.

The insulator assembly 22, in the preferred embodiment illustrated, comprises a metal, three pronged spider 75 having a central bore lla for close fitting engagement with the exterior of the support member l2 and is fastened to same by means of set screws 72. Each of the spider prongs Nib is provided with recesses H30 for receiving the lower ends of elongated pillars M of insulating material, preferably of the type and quality known as Textolite, manufactured and supplied by the General Electric Company. The pillars are secured to the spider bracket by means of screws I6. Received on the upper ends of the pillars is an annular, washer-like member 78, also made of insulating material, preferably polystyrene or Textolite. l'his member is secured to the pillars by means of screws 3t and has three circumferentially spaced projections 18a on its inner circumference for engaging the skirt M in centering relation near its lower end. Because of its centering function, it is herein denominated a yoke. the projections 13a are preferably equi spaced.

While the annular, washerlike member 18 may conceivably be made without the inwardly presenting projections 78a, they are a very desirable part of the insulating assembly, inasmuch as they serve to reduce to a minimum the area of contact of the insulator assembly with the skirt, and are, therefore, by their smallness, an important part of the resistance path. They likewise present a very limited area on which electrically conducting films may form.

It will thus be seen that the assembly comprising the spider l0, pillars I l and yoke 78 serves to hold the support member and skirt in concentric relation to each other, with no connection between the interior of the skirt and the exterior of the support member. There is thus avoided any trap surface between these two members upon which corrosive substance or electrolyte can form to provide an electrical leak from the support member to the skirt. Inasmuch as the piliars may be of optional length, any degree of insulation may be obtained by an insulator of this type, and also the path of leakage due to film formation on the pillars may be greatly lengthened, thereby to increase its insulation resistance.

It will be noted that the insulator assembly lends itself to the general objective of facilitating assembly and disass-smbly of the antenna as a whole. For instance, when it is desired to disassemble the antenna to cleanse the same, the entire insulator assembly 22 may be slipped down the support member, as a unit, by loosening set screw 12, and then the skirt may be disconnected from the head assembly by removal of the screws 3%. If desired, the insulator assembly can, of course, be completely disassembled by removal of the screws '16 and 80.

It will be understood by those skilled in the art that the spider may also be made of insulating material, if desired; that the yoke may be made of metal, if desired; that the spider and yoke may be reversed, if desired, with the spider being attached to the skirt; that the yoke and pillars may be cast in an integral piece; that any number of pillars may be used, from one to a solid formation resulting in a cylinder, so long as sufficient rigidity is provided; that the yoke may be eliminated and the pillars alone used as structural centering members for the skirt, as by curving their upper ends into contact with the skirt, or by aligning them with their inwardly presenting sides in contact with the outer surface of the skirt wall; and that any number of other modifications or alterations may be made, so long as the essential consideration is observed of avoiding a bridging structure between the inner wall of the skirt and the support member.

Various modifications for the construction of the head assembly it will also occur to those skilled in the art for practicing the teaching of this invention of making the skirt detachable from the support member to facilitate assembly and disassembly of the antennae. For instance, the bushing member 36 may be made of non-metallic material, if an electrical path is provided between the shielding braid 44, the skirt I4, and the support member I2.

It is also to be understood that the terms concentric, coaxial and concentricity are used herein to include as equivalents a constructional relationship of uniform laterally spaced members of any shape cross-section and relative disposition to each other, were the support member or skirt to be made of other than circular cross-section.

Having thus described the invention, what I desire to protect by Letters Patent is as defined in the following claims:

1. A dielectric spacer assembly for a free open lower end of a coaxial antenna skirt member surrounding a vertical support member of a whip type antenna comprising a spider having an apertured hub to receive one of said members, means to secure said hub rigidly to said received member, aelongated, insulators on :said spider, a.- yoke carried by said insulators to,-encircle the-other :of said members, and inwardly directed projections insaid .yoke to engage'said other member exteriorly at spaced points.

2. Aqdielectric spacer assembly for a free open end of a coaxial antenna skirt surrounding an antenna support member comprising a spider, having a hub to receive said support member,-means to secure said hub rigidly to-said support member, elongated insulating members onsaid spider, a;yoke carriedby said insulating members to encirclesaid skirt, and inwardly directed projectionsinsaid yoke to engagesaidskirt exteriorly at spaced points.

3. A dielectric spacer assembly as defined in claim 2, said spider having a plurality of arms, arecess in the extremity of each armseating one of said insulating members, said insulating members comprising pillars extending parallel with said support member and skirt.

4. A;die1ectric spacer assembly ,asdefined in claim 2 Whereinsaid insulating members are connected: with said yoke at uniformly spaced points and said projectionsin said yoke are disposed midway between said spaced points.

v5. A dielectric spacer-assembly as defined in claimv 2 wherein said insulating members are three in, number and extend parallel with said support member and skirt, and said projections in said yoke. are also three in number.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,969,911 Smith Aug. 14, 1934 2,029,421 Green et a1 Feb. 4, 1936 2,078,234 Buschbeck Apr.,27,,1937 2,184,729 Bailey Dec. 26, 1939 2,191,071 Duttera Feb. 20, 1940 2,376,725 Richardson et a1. May. 22, 1945 2,451,258 Trevor Oct. 12,1948 2,508,438 Wilson et al May 23,1950 2,513,336 Lewis July 4,1950 

